This invention relates to rotary shaft angular position sensors and, more specifically, to contactless angular sensors adapted to produce precise, linear output signals proportional to shaft position for a wide range of rotational input angles, and associated methods. 2. Description of Prior Art
Shaft angular position sensing such as in rotary motion control and other applications, has been traditionally accomplished using potentiometers, synchros, or resolvers which include low reliability electrical contact arrangements such as electrical brushes and wipers. Newer technologies for angular position sensing includes optical encoders which are unreliable in low temperature, moist environments. The need for high reliable shaft angle sensing for aircraft control surfaces and closed loop actuators has led to the application of rotary variable differential transformers and brushless resolvers. Unfortunately, these sensors are substantially more expensive, they require AC excitation and demodulation electronics to obtain useable output signals, resulting in additional complexity and cost, and they do not provide for inherent digital signals that are desired for most modern automation systems.
Hall-effect and magnetoresistive (MR) sensors are relatively low cost sensors capable of generating an electrical output signal when exposed to a rotating magnetic field. Hall-effect sensors utilize a current-carrying semi-conductor membrane to generate a low voltage perpendicular to the direction of current flow when subjected to a magnetic field normal to the surface of the membrane. Magnetoresistive sensors utilize an element whose resistance changes in the presence of a changing external magnetic field. Hall-effect and magnetoresistive sensors have been historically limited to use for sensing motion over a limited angular range where a high degree of accuracy is not required. However, recent efforts to achieve a lower-cost, yet reliable and accurate device for sensing angular position of a rotary shaft have included attempts to utilize relatively low cost sensors such as Hall-effect devices or magnetoresistive sensors.
One group of prior devices to utilize these magnetic field sensors provides an output which is digital in nature, generating pulses as a function of shaft rotation or discrete signals for incremental shaft angles. Nichols, U.S. Pat. No. 4,373,486, Schroeder, U.S. Pat. Nos. 5,731,702 and 5,754,042, and Seefeldt, U.S. Pat. No. 5,744,950, use permanent magnet biased Hall-effect devices and magnetoresistive sensors, respectively, to sense the passage of notches on a shaft-driven wheel for engine ignition control. Kajimoto, U.S. Pat. No. 5,574,364, utilizes magnets imbedded into or polarized into the surface of the rotating wheel to provide a changing magnetic field direction as the surface of the wheel passes the sensors. The accuracy of such digital output signals is limited by the number of pulses per revolution of the wheel or shaft.
Another group of prior devices utilize these magnetic field sensors to provide analog output signals as a magnet attached to a shaft is rotated. van den Berg, U.S. Pat. No. 5,650,721, shows a two-pole rectangular bar magnet rotating over a giant MR layer. The rotation of the transverse field between the poles creates a unique, sine-wave-shaped analog output over 180 degrees of rotation. Linear output range is less than 60 degrees. Lochmann, U.S. Pat. No. 6,064,197, adds a Hall-effect device to sense axial field direction to provide a unique, but nonlinear, signal over 360 degrees. Andraet, U.S. Pat. No. 5,796,249, proposes the integration of at least three MR Wheatstone bridges under a bar magnet's transverse field to provide a set of nonlinear outputs that can be used to calculate a unique shaft angle. Haberli, International Publication WO98/54547, proposes a similar scheme utilizing two pairs of Hall-effect sensors located on diagonals under a square magnet to generate approximate sine and cosine signals as the shaft and magnet are rotated. Muth, U.S. Pat. No. 5,602,471, proposes use of multiple MR bridges to generate a variety of phase-spaced sinusoidal signals. The signals are forced to saturate within their linear range and then added to provide a summed output which is overall a linear function of shaft rotations, but can exhibit a variety of gain variations and discontinuities. None of these analog sensors are capable of precisely measuring shaft angle over a wide range of angles, or lend themselves to being compactly packaged around an axially continuing shaft, a feature desirable for integrating the sensor into a gearbox or actuator.
Other prior analog shaft angle sensors using magnetic flux sensors have attempted to increase the accuracy and linear range by shaping the magnets or pole pieces. Wu, U.S. Pat. No. 5,159,268, has generated a bell or oblong shaped two-pole magnet to get a linear range approaching 180 degrees. Rountos, U.S. Pat. No. 5,850,142, uses a pair of convex magnets and a spherical pole piece to generate a linear range of up to plus and minus 30 degrees for joysticks. Dawley, U.S. Pat. No. 4,719,419, uses a monopolar annular magnet, either mounted eccentric to the shaft or nonuniformly magnetized, to create a useable linear output of +45 degrees. Nakamura, U.S. Pat. No. 4,425,557, and Tomczak, U.S. Pat. No. 4,570,118 incline the sensor magnets relative to the axis of rotation in an attempt to improve output linearity. Luetzow, U.S. Pat. Nos. 5,444,369 and 6,137,288 and Herden, U.S. Pat. Nos. 5,861,745 and 6,130,535 use a combination of shaped magnets, pole pieces, and axis offsets to get a linear output range approaching 180 degrees.
Overall, the prior contactless shaft sensing devices provide a limited number of discrete pulses or have a small linear output range. They are often manufactured utilizing non-standard magnetic shapes and cannot be easily packaged around a rotating shaft. They do not provide the accuracy of resolvers or RVDT's.
Thus, it is apparent there is a need for a new rotary shaft sensor that is economical to manufacture, can be compactly packaged, can measure the shaft angle of rotation to a high degree of accuracy, and is readily adaptable to a wide range of input angles.